A bipolar junction transistor (BJT) uses both electron and hole charge carriers. The BJT may be manufactured in two types, NPN and PNP, and may be used as, for example, an amplifier or a switch for variety of electronic devices. An NPN BJT includes two regions of n-type semiconductor material constituting the emitter and collector regions, and a region of p-type semiconductor material located between the two regions of n-type semiconductor material constituting the base region. A PNP BJT consists one region of N-type semiconductor material between two regions of P-type semiconductor material. A heterojunction bipolar transistor (HBT) is a type of BJT which uses differing semiconductor materials for the emitter and base regions, creating a heterojunction, and may operate based on a bandgap difference between the emitter and the base. For example, the emitter may include silicon, and the base may include silicon germanium (SiGe). The SiGe HBT may be superior than the conventional silicon BJTs in many aspects including, for example, reduction in base-transit time resulting in higher frequency performance, an increase in collector current density and hence higher current gain, and an increase in early voltage at a particular cutoff frequency.
The structure or composition of the SiGe base may be tailored to further enhance the performance of the SiGe HBT. For example, the base transit time can be further reduced by building into the base a drift field that aids the flow of electrons from the emitter to the collector. There are two ways of accomplishing this. The classical method is to use graded base doping, i.e., a large doping concentration near the emitter-base (EB) junction, which gradually decreases toward the collector-base (CB) junction. The other approach is to have the energy gap of the base decreasing from the emitter end to the collector end. In other words, the germanium (Ge) content of the SiGe in the base of the HBT is higher on the collector side. There are many technical challenges in forming the base with graded Ge content with high Ge content at the CB junction. For example, high Ge content strained SiGe cannot be epitaxially grown on Si without defects. In other words, the SiGe layer with a certain amount of Ge content may require a thickness higher than a certain thickness for the device to function well, however, this required thickness may be higher than the critical thickness that the defect free SiGe layer with this Ge content may be epitaxially grown. To alleviate this problem, a conventional Ge condensation on silicon on insulator (SOI) process may be used to increase the Ge content in the SiGe layer. The conventional Ge condensation process may require high temperature. The high temperature (>950° C.) oxidation process required may not be compatible with the Si/SiGe dual channel complementary metal-oxide-semiconductor (CMOS) device. In addition, Ge out diffusion cannot be controlled at this high temperature. As such, there is a need for providing a method in which a BJT structure with SiGe base having high Ge content can be fabricated without being subjected to the high temperature Ge condensation process.